(Originally published by the University of Cambridge)
January 6, 2014
Figure 1: Computer simulations of the large-scale matter distribution in cold dark matter (left) and warm dark matter (right) Universes. The streaming motions of warm dark matter suppress structure formation on small scales. (Credit: KICC)
Current cosmological models such as Lambda-CDM (“Lambda Cold Dark Matter”) generally assume that dark matter particles are kinematically “cold”. The low random motions of these particles would allow low-mass dark matter halos to form in abundance via gravitational collapse. In a Lambda-CDM cosmology, therefore, a massive halo such as the one that hosts the Milky Way is predicted to be surrounded by numerous low-mass halos, many of which should also contain stars. Current observations, however, have uncovered fewer such satellite galaxies than are naively predicted for Lambda-CDM. To solve this “missing satellite problem”, some groups have proposed that dark matter may not be cold after all, but instead “warm” enough for the random motions of the dark matter particles to prevent low-mass halos from forming.
To address this possibility, a team of astronomers including KICC’s George Becker and Martin Haehnelt tested the predictions of a warm dark matter cosmology not by counting satellite galaxies around the Milky Way, but by looking at the distribution of matter in the early Universe. The team used the “Lyman-alpha forest” in the spectra of some of the most distant known quasars to map the small-scale structure of the gas pervading deep space roughly one to three billion years after the Big Bang. The measurements were then compared to predictions from computer simulations of cold dark matter and warm dark matter Universes.
The results showed that the gas distribution in deep space contains far too much small-scale structure to be compatible with the type of warm dark matter that has been proposed to solve the small-scale crisis. It is fully consistent, however, with expectations for cold dark matter. The team noted that looking at the gas in deep space provides a cleaner test of warm dark matter than counting satellite galaxies around the Milky Way, as stars may not form in the lowest-mass halos due to several factors, leaving many of these halos invisible and unaccounted for. With the simplest forms of warm dark matter ruled out, therefore, other explanations for the missing satellite problem may be needed.
The work was authored by Matteo Viel at Trieste Observatory, Italy, and co-authored by George Becker and Martin Haehnelt of KICC, along with James Bolton at the University of Nottingham. Paper Reference: Viel, Becker, Bolton, & Haehnelt 2013, PhysRevD, 88, 3502.